Process for recovering organic acids from bleaching liquors

ABSTRACT

The invention relates to a process for treating bleaching liquors from pulping processes based on organic acids. The process is characterized in that inorganic acid, such as nitric acid, phosphoric acid and/or sulphuric acid, is added into a bleaching liquor obtained from the bleaching or a stage closely associated with bleaching, whereby organic acids contained in the bleaching liquor are released into the bleaching liquor. The organic acids are then separated from the bleaching liquor and recycled to the cooking stage to be used as cooking chemicals. The nitrogen-, phosphor- and/or sulphur-containing product thereby produced, from which organic acids have been separated, is recovered to be used as a fertilizer or as raw material for a fertilizer.

BACKGROUND OF THE INVENTION

The invention relates to pulping processes based on organic acids andparticularly to the recovery of organic acids from bleaching liquors,such as alkaline filtrates and washing filtrates, obtained frombleaching or from stages closely associated with bleaching. At the sametime, the process of the invention produces a valuable fertilizerby-product. The invention further relates to recycling of bleachingliquors. The invention is applicable in pulping processes based onorganic acids, especially formic acid and/or acetic acid, whereinherbaceous plants or trees are used as raw material. The invention isalso applicable in pulping processes employing other solvents andreagents.

Organic acids are produced in all pulping processes, but theirutilization remains unsolved. In conventional processes, organic acidsare usually harmful and consume the alkali used in the processes.Organic acids are formed not only in cooking, evaporation and drying oflignin, but also in bleaching. In alkaline hydrogen peroxide and/oroxygen/hydrogen peroxide bleaching, formic acid, acetic acid, oxalicacid and glycolic acid are typically produced. These acids react in thebleaching with alkalis to corresponding alkali metal salts. They add tothe load of organic matter (COD) in waste waters and to the use ofchemicals in water purification plants. The same problem appears inclosed bleaching, because the salts do not evaporate, but remain in theconcentrate. The treatment of the concentrate therefore continues to bean environmental and waste water issue.

FI patent publications 103 588 (WO 96/35013) and 103 899 (WO 98/20198)disclose processes based on formic acid which make use of the acidsformed in cooking, evaporation and drying of lignin, the organic acidsthereby formed being advantageous in view of process economy becausethey save cooking chemicals. Organic acids are formed as a result ofhydrolysis reactions of hemicellulose and lignin and from acetyl groupsof plants. In highly acid concentrations, esters are also produced.Esters may be converted back to organic acids, as described in FI patentapplication 973474 (WO 99/10595). However, these processes do not takeadvantage of the organic acids formed in the bleaching; on the contrary,the acids are lost in the alkaline bleaching, because they reactimmediately with the sodium hydroxide used in the bleaching, wherebysalts are formed. Such salts are undesirable also in conventionalprocesses because they get into waste waters.

BRIEF DESCRIPTION OF THE INVENTION

It is therefore an object of the present invention to provide animproved process based on organic acids for preparing pulp fromherbaceous plants and trees in a manner which allows also the organicacids included in the bleaching liquors to be utilized. This allows anyenvironmental and waste water problems involved in organic acids to beavoided and all the chemicals used in the process to be either returnedto the process or utilized as commercial products. The process operateswith a completely closed chemical and water cycle.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for treating bleaching liquors inpulping processes based on an organic acid/organic acids. The processcomprises cooking and bleaching of pulp in a manner known per se. Thecooking is carried out using an organic acid, typically formic acid oracetic acid, a combination thereof, or a mixture comprising both acidsor one of them. The bleaching is carried out using chlorine-freebleaching processes known per se. The raw material of the pulp comprisesherbaceous plants or trees.

The invention is based on the idea of

(a) adding inorganic acid, such as nitric acid, phosphoric acid and/orsulphuric acid, to bleaching liquor obtained from bleaching or a stageclosely associated with bleaching, in order to release organic acidscontained in the bleaching liquor into the bleaching liquor;

(b) separating the organic acids from the bleaching liquor and recyclingthe same to cooking to be used as cooking chemicals; and

(c) recovering the nitrogen-, phosphor- and/or sulphur-containingproduct thus obtained, from which organic acids have been separated, tobe used as a fertilizer or as raw material for a fertilizer.

In this context, the expression “bleaching liquor obtained frombleaching or a stage closely associated with bleaching” refers to aliquor obtained from the actual bleaching stages, pre-bleaching stagesor pulp screening, or from any washing stages associated with these,after the pulp itself has been separated. The term “bleaching filtrate”means any bleaching liquor obtained from a stage associated with theactual bleaching, for example a bleaching liquor obtained in connectionwith alkaline hydrogen peroxide bleaching from the alkaline extractionstage or peroxide stage after the separation of pulp.

Before stage (a), the bleaching liquor may be concentrated, typically byfirst filtering and then evaporating it to a dry matter content of 10 to75%, for example. The evaporated water is used as process water. Thebleaching liquor may, however, also be used in a non-concentrated form.

The silicon compounds in herbaceous plants may be precipitated beforestage (a) by adjusting the pH of the bleaching liquor to a range of 7 to9. This may be carried out using the same inorganic acid as in stage(a), i.e. nitric acid, phosphoric acid and/or sulphuric acid. Theprecipitated silicon compounds are separated and the bleaching liquorobtained is concentrated, typically by evaporating it to a dry mattercontent of 10 to 75%. The evaporated water is used as process water. Theseparation of the silicon compounds before the evaporation improves theevaporation conditions. At the same time, the content of formiate andacetate salts in the concentrate increases, which allows as high acidconcentrations in stage (a) as possible to be ensured. However, anon-concentrated bleaching liquor may also be used.

The inorganic acid used in stage (a) of the process of the invention isselected on the basis of the desired nutrient content of the fertilizerby-product to be obtained. If a nitrogen-containing fertilizer is to beobtained, nitric acid is used, if a phosphor-containing fertilizer is tobe obtained, phosphoric acid is used, and if a sulphur-containingfertilizer is to be obtained, sulphuric acid is used.

The pH range of the filtrate leaving the alkaline bleaching stage istypically 7 to 11.5 (when the bleaching is carried out for example as analkaline hydrogen peroxide bleaching where potassium hydroxide is usedas the alkali). In stage (a) of the process of the invention, inorganicacid is added to the bleaching liquor to reduce its pH to a level whereorganic acids are released for example from their esters or salts to thebleaching liquor.

In an industrial implementation of the process, the inorganic acid addedin stage (a) is as concentrated as possible to minimize the amount ofwater supplied together with the acid. The concentration of the acid maybe for example 60 to 95%, although less concentrated acids can also beused in the present invention. The amount of the inorganic acid to beadded is such that the pH range of the bleaching liquor is typically 0to 6, preferably 0 to 3.

To obtain a nitrogen-containing fertilizer, nitric acid alone is usedfor adjusting the pH of the bleaching liquor to the range of 0 to 6,preferably 0 to 3.

A fertilizer containing nitrogen and phosphor can be obtained forexample by adding first phosphoric acid to provide a neutral pH, i.e. ofabout 7 to 8. Then, nitric acid is added to provide the pH of about 0 to6, preferably 0 to 3.

When an inorganic acid is used to acidify the bleaching liquor in stage(a) of the process of the invention, the formiates and acetatescontained in the bleaching liquor are released and corresponding organicacids, i.e. formic acid and acetic acid, are formed. The formiates andacetates formed from the washing losses of pulp react similarly, organicacids (formic acid and acetic acid) being thereby formed into thesolution. In other words, organic acids are released using inorganicacids which are stronger than the organic ones, inorganic salts beingformed at the same time. As mentioned above, the acids in question areparticularly acids producing salts that can be used as fertilizers.

The organic acids released into the bleaching liquor are separated fromit, typically by means of evaporation or extraction and/or drying, whichallows the inorganic salts to separate at the same time bycrystallizing. The evaporation may be carried out using a thin filmevaporator/drier, for example. The organic acids are then usuallyconcentrated by distillation to provide the desired acid concentrationafter which they are recycled to cooking to be used as a cooking acidand/or sold as a commercial product.

It has been observed that the amounts of organic acids thus recoveredare significant with regard to the total amount of chemicals needed. Thetotal amount of formic acid, acetic acid and oxalic acid that can beseparated from the bleaching liquor may be as much as 6 to 8% (60-80 kgper 1000 kg) of dry pulp. The total amount of make-up cooking chemicalsis typically 5 to 10% (50-100 kg per 1000 kg) of pulp.

The nitrogen-, phosphor- and/or sulphur-containing product obtained fromstage (b), from which organic acids have been separated, is recovered instage (c) to be used as a fertilizer or as raw material for afertilizer. The product obtained from stage (c) is usually alsoconcentrated. The concentration is typically carried out by evaporation,drying and/or crystallization to a dry matter content of 80 to 100%, forexample.

The final fertilizer is typically produced for example by granulatingthe nitrogen-, phosphor- and/or sulphur-containing product thus obtainedto convert it into a solid form suitable to be used as a fertilizer.

The nutrient value of the fertilizer may be further increased if thebleaching that produces the bleaching liquor in stage (a) is carried outby using a bleaching sequence comprising alkaline stages and,particularly, by using potassium hydroxide as the alkali, as it willproduce potassium, which is most important for plants, into thebleaching liquor.

The bleaching liquor, typically a bleaching filtrate, used as thestarting material in stage (a) originates therefore preferably from ableaching sequence containing alkaline stages. The bleaching istypically an alkaline hydrogen peroxide bleaching. The bleachingsequence may also comprise an oxygen phase, ozone phase or peraceticphase.

The alkali used in bleaching sequences comprising alkaline stages ispreferably potassium hydroxide, either alone or together with sodiumhydroxide and/or ammonium hydroxide. Potassium hydroxide can also beused for increasing pH before the bleaching. In addition to potassiumhydroxide, other potassium compounds may also be used. This allows apotassium-containing bleaching liquor rich in potassium nutrients neededby plants to be obtained.

In addition to potassium, magnesium compounds may also be used inconnection with bleaching since magnesium compounds also have nutrientvalue in fertilizer use. The most common way to use magnesium in pulpbleaching is to use magnesium sulphate as a stabilizer. Magnesium in theform of Mg(OH)₂ can also be used before the bleaching to increase the pHof acidic pulp. At the same time, it serves as a stabilizer inoxygen-based bleaching stages, for example in hydrogen peroxide andperacetic bleaching.

However, bleaching comprising alkaline stages can also be used togetherwith ozone or per(carboxylic)acid bleaching, the fertilizer produced asa by-product naturally being then free of potassium.

As a result of bleaching in which potassium hydroxide is used and inwhich phosphoric acid serves as the inorganic acid in stage (a), formicacid, acetic acid and potassium phosphate are produced. Formic acid isreturned to cooking and acetic acid is either returned to cooking and/oroffered for sale. Potassium phosphate is recovered to be used incompound fertilizers, for example (potassium phosphate being animportant part of compound fertilizers).

Correspondingly, when the bleaching is carried out using potassiumhydroxide and nitric acid is used as the inorganic acid in stage (a),formic acid, acetic acid and potassium nitrate are produced. The lastone is an important nitrogen-potassium fertilizer. Potassium hydroxide,phosphoric acid and nitric acid used together allow an NPK fertilizer tobe prepared.

The inorganic acid used in stage (a) may also be sulphuric acid whichtogether with potassium hydroxide produces potassium sulphate used inchlorine-free fertilizers. Chlorine-free fertilizers are useful forexample in the preparation of high-quality garden fertilizer products.

The composition of the fertilizer can be further adjusted by adding ashobtained from a bio power plant of a pulp mill to the potassium-,nitrogen-, phosphor- and/or sulphur-containing bleaching concentratethus produced. The lignin fraction recovered from the cooking andwashing stage and, together with it, possibly other organic matterproduced as a by-product of the pulp production are burned in the powerplant. Said ash fraction contains nutrients (K, Ca, P, Mg, Fe and otheracid-soluble components) which have been separated during the cookinginto the cooking liquor and further into the lignin-hemicellulosefraction during the recovery of formic acid used as the cookingchemical. The ashes may also contain an ash component obtained from theburning of cellulose raw material, such as straw screenings.

In hydrogen peroxide bleaching, the hemicellulose contained in the pulpreacts from 1 to 5 percentage units, depending on the bleachingrequirements. Pulp typically contains about 15% of hemicellulose, theproportion of unreacted hemicellulose left in the pulp after thebleaching amounting up to 10-14%. The hemicellulose obtained from plantsand hardwoods mainly consists of xylans that react to xylose, afterwhich they are hydrolyzed to form organic acids. Similarly, some of thelignin separated in the bleaching forms organic acids. The process ofthe invention allows such organic acids to be recovered.

Moreover, the process of the invention allows to utilize any free acidsand formiate and acetate esters possibly left as washing loss in theunbleached pulp, because they react with the alkaline bleaching chemicalto corresponding salts. Also these salts react in the acid treatment toorganic acids, the process of the invention thus allowing them to bereused, which significantly reduces the losses in the washing ofunbleached pulp, the losses being 0.5 to 5% of pulp, depending on theprocess applied.

The concentrate produced by the evaporation of the bleaching liquorsalso contains other organic compounds, such as lignin and hemicellulose.These compounds are included in the bleaching concentrate obtained inaccordance with the invention, the invention thus allowing the compoundsto be utilized as additional components in the fertilizer.

The above process operations allow a fully closed water cycle to beimplemented in a pulp mill. The organic acids formed in the pulpingprocess can be utilized as cooking chemicals, and the hemicellulosedissolved in the bleaching can be completely hydrolyzed into organicacids which in turn can be utilized in the cooking. The evaporated wateris reused as process water. The acids formed in the process can be usedto compensate for the chemical losses occurring in the process. In anoptimum situation, the overall consumption of cooking chemicals in theprocess may be compensated for by using acids formed in the process, andall inorganic chemicals may be utilized as a fertilizer. The consumptionof cooking chemicals in pulping processes based on organic acids istypically 5 to 10% (50-100 kg per 1000 kg) of pulp.

The process of the invention allows a total amount of organic acids of 6to 8% (60 to 80 kg per 1000 kg) of dry pulp to be obtained.Correspondingly, salts suitable for use as fertilizers are produced inan amount of 12 to 16% (120 to 160 kg), of which potassium, nitrogen andphosphor (K, N, P) account for 50% (the bleaching being carried outusing potassium hydroxide and the inorganic acids used in stage (a)being nitric acid and phosphoric acid).

The process of the invention is particularly suitable for pulpingprocesses based on formic acid and/or acetic acid and using herbaceousplants and corresponding non-wood fibre sources as raw material. Themost important fibre sources are straw, typically corn straw (wheat,rye, oats, barley, rice), grasses, e.g. giant reed, esparto grass, sabaigrass and lemon grass, reeds, such as papyrus, common reed, sugar cane,or bagasse, and bamboo, bast fibres, e.g. stems of fibre flax or seedflax, kenaf, jute and hemp, leaf fibres, e.g. manilla hemp and sisal,and seed hairs, such as cotton and cotton linter fibres.

Useful grasses that grow in Finland are e.g. common reed, reed canarygrass, tall fescue and timothy.

The process of the invention can also be applied to wood material.

The invention applies not only to processes based on organic acids, suchas formic acid, but also to pulping processes using other solvents andreagents.

A closed bleaching cycle is particularly well suitable for the processof the invention and for other processes employing acidicdelignification and pulp washing stages. For example, calcium (Ca) isdissolved and washed from the pulp and not taken to bleaching, whereoxalic acid is formed in all bleaching sequences. If calcium is presentin the bleaching, as for example in kraft and soda processes and othersimilar processes, calcium oxalate is formed, which is known to bepoorly soluble and which may precipitate even quite firmly to filtratecontainers, wires and on the surfaces of evaporators. In the process ofthe invention the calcium contained in the raw material is separated inthe cooking and pulp washing, wherefore it cannot form calcium oxalatein the bleaching liquors. The calcium ends up in the ash produced fromthe burning of the lignin.

The process of the invention can be applied in sulphate processes andother similar processes, provided that the impact of calcium has beeneliminated by an acidic washing stage having pH of 3 or lower. Organicacids can then be utilized in a sulphate process to adjust the pH in therecovery of tall oil, for example, which allows excessive amounts ofsulphur (extra sulphur load) in the process to be avoided by replacingsulphuric acid with organic acids.

The following examples illustrate the invention:

EXAMPLE 1

Cellulose derived from wheat straw and produced in a formic-acid-basedprocess was used (the amounts of bound acids washed in unbleached pulpbeing: formic acid 0.9% and acetic acid 2.4%). The unbleached pulp wasbleached in an alkaline hydrogen peroxide bleaching using KOH to adjustthe pH. The amount of hydrogen peroxide used was 3.0% and that of KOH11.9% of the dry pulp. From the alkaline peroxide bleaching filtrate ofthe pulp (having initial consistency of 12%, final pH 10.9) a batch of71.4 g was taken and neutralized with 0.5 M nitric acid to a pH of 2.The amount of nitric acid consumed was 52.4 ml, or 0.03 mol, the nitricacid consumption (calculated as 100% nitric acid) per 1000 kg ofbleaching filtrate being 23.1 kg (367 mol).

The fertilizer amounts per 1000 kg of bleaching filtrate were: potassium(K) 10.0 kg and nitrogen (N) 4.1 kg. The fertilizer amounts per 1000 kgof dry pulp were K=84 kg and N=35 kg.

The acid contents (the amounts of organic acids delivered to the acidrecovery stage) per 1000 litres of bleaching filtrate and 1000 kg of drypulp were as follows:

Acid kg/1000 l of filtrate kg/1000 kg of pulp Oxalic acid 1.3 13.3Glycolic acid 2.3 20.4 Formic acid 2.4 21.4 Acetic acid 4.0 35.4

EXAMPLE 2

Cellulose derived from Miscanthus sinensis and produced in aformic-acid-based process was used (the amounts of bound acids washed inunbleached pulp being: formic acid 1.2% and acetic acid 2.4%). Theunbleached pulp was bleached in an alkaline hydrogen peroxide bleachingusing KOH to adjust the pH. The amount of KOH used in the alkalineextraction stage was 8.4% and that of hydrogen peroxide 0.75% of theamount of dry pulp. After the alkaline extraction stage (having aninitial consistency of 12%), two hydrogen peroxide stages (having aconsistency of 10%) were carried out, the amount of hydrogen peroxideused in the first stage being 2.5% and in the second stage 2.0% of thedry pulp amount. From the bleaching filtrate (having a final pH of 11.1)obtained from the alkaline extraction stage of the pulp bleaching, abatch of 100 g was taken and neutralized with 0.8 M phosphoric acid topH 7 and with 0.5 M nitric acid from pH 7 to pH 2. The amount ofphosphoric acid (H₃PO₄) consumed was 9.2 ml (0.007 mol) and that ofnitric acid 52.0 ml (0.03 mol), the consumption of nitric acid and thatof phosphoric acid (calculated as 100% acids) per 1000 kg of bleachingfiltrate being 6.9 kg (71 mol) for the phosphoric acid and 12.3 kg (195mol) for the nitric acid (HNO₃).

The fertilizer amounts per 1000 kg of bleaching filtrate were: potassium(K) 7.0 kg, phosphor (P) 2.3 kg and nitrogen (N) 2.8 kg. The amounts offertilizer per 1000 kg of dry pulp were K=62 kg, P=20 kg and N=25 kg.

The following table shows the amounts of organic acids recovered fromthe different bleaching stages, i.e. from the alkaline extraction andthe first and second peroxide stages, per 1000 litres of bleachingfiltrate:

Alkaline extraction Peroxide 1 Peroxide 2 Total (kg/1000 l) (kg/1000 l)(kg/1000 l) (kg/1000 l) Oxalic acid 0.5 0.8 0.2 1.5 Glycolic acid 0.61.7 1.0 3.3 Formic acid 1.9 0.7 0.4 3.0 Acetic acid 3.4 0.8 0.3 4.5Total 6.4 4.0 1.9 12.3

The following table shows the amounts of organic acids recovered fromthe different bleaching stages, i.e. from the alkaline extraction andthe first and second peroxide stages, per 1000 kg of dry bleached pulp:

Alkaline extraction Peroxide 1 Peroxide 2 Total (kg/1000 kg) (kg/1000kg) (kg/1000 kg) (kg/1000 kg) Oxalic acid 6.0 8.1 2.1 16.2 Glycolic acid7.2 17.2 10.4 34.8 Formic acid 22.7 7.1 4.2 34.0 Acetic acid 40.5 8.13.1 51.7 Total 76.4 40.5 19.8 136.7

It is apparent to a person skilled in the art that as technologyadvances, the basic idea of the invention can be implemented in variousways. The invention and its embodiments are therefore not restricted tothe above described examples but they may vary within the scope of theclaims.

1. A process for treating bleaching liquors from pulping processes basedon organic acids, the process comprising cooking with organic acids andbleaching of pulp, and further comprising (a) adding inorganic acidselected from nitric acid, phosphoric acid and/or sulphuric acid tobleaching liquor obtained as a result of bleaching or a stage closelyassociated with bleaching; (b) separating the organic acids from thebleaching liquor and recycling the same to cooking to be used as cookingchemicals; and (c) recovering the nitrogen-, phosphor- and/orsulphur-containing product thus obtained, from which organic acids havebeen separated, to be used as a fertilizer or as raw material for afertilizer.
 2. A process according to claim 1, wherein the pulp isbleached using alkaline bleaching in which at least some of the alkaliis potassium hydroxide.
 3. A process according to claim 2, wherein alsomagnesium hydroxide and, optionally, ammonium hydroxide are used as thealkali.
 4. A process according to claim 1, wherein the bleaching liquorto which the inorganic acid is added in stage (a) is a bleachingfiltrate.
 5. A process according to claim 1, wherein the amount ofinorganic acid to be added in stage (a) is such that it allows ableaching liquor pH within a range of 0 to 6 to be obtained.
 6. Aprocess according to claim 5, wherein a bleaching liquor pH within arange of 0 to 3 is obtained.
 7. A process according to claim 1, whereinnitric acid is added in stage (a) to adjust the pH of the bleachingliquor to a range of 0 to
 6. 8. A process as claimed in claim 7, whereinpH is adjusted to a range of 0 to
 3. 9. A process according to claim 1,wherein phosphoric acid is added in stage (a) to adjust the pH of thebleaching liquor to a range of 7 to 8 after which nitric acid is addedto adjust the pH of the bleaching liquor to the range of 0 to
 6. 10. Aprocess according to claim 9, wherein nitric acid is added to adjust thepH of the bleaching liquor to a range of 0 to
 3. 11. A process accordingto claim 1, wherein in stage (b) organic acids are separated byevaporation or extraction.
 12. A process according to claim 1, whereinthe nitrogen-, phosphor- and/or sulphur-containing product obtained fromstage (c) is converted into a solid form to be used as a fertilizer. 13.A process according to claim 12, wherein the product is granulated. 14.A process according to claim 1, wherein the organic acid used in thepulp production comprises formic acid, acetic acid or a mixture thereof.15. A process according to claim 1, wherein before stage (a) thebleaching liquor is concentrated by evaporation and the evaporated wateris used as process water.
 16. A process according to claim 15, whereinthe bleaching liquor is concentrated to a dry matter content of 10 to75%.
 17. A process according to claim 1, wherein the inorganic acidadded in stage (a) is added into a non-concentrated bleaching liquor.18. A process according to claim 1, wherein the raw material used forthe pulp comprises herbaceous plants.
 19. A process according to claim18, wherein stage (a) is preceded by a stage in which the pH of thebleaching liquor is adjusted to a range of 7 to 9 to precipitate siliconcompounds in one or more stages, the precipitated silicon compoundsbeing separated and the bleaching liquor thus obtained concentrated. 20.A process according to claim 19, wherein the bleaching liquor isconcentrated by means of evaporation to a dry matter content of 10 to75%.